1. Field of the Invention
The present invention relates to a silver salt photothermographic dry imaging material (hereinafter, also referred to as “photothermographic imaging material”) with low photographic fog, high sensitivity and high maximum density, which are good in color tone and excellent in rapid thermal development suitability, and an image recording method and an image forming method using the same.
Further, The present invention relates to a photothermographic imaging material, and particularly a photothermographic imaging material with high density which are excellent in light radiated image stability, silver color tone, changes of silver color tone with time, density unevenness at thermal development and image storage stability in storage at room temperature.
2. Description of Related Art
Recently, in the fields of medical care and print plate making, waste solutions involved in wet processing of image formation materials have been problematic in terms of working property, and reduction of processing waste solutions has been strongly desired in the light of environmental preservation and saving space. Thus, technology concerning photothermal photographic materials for photographic technology use such as laser imagers and laser image setters where efficient exposure is possible and clear black images with high resolution can be formed has been required.
As the technology according to the above photothermal photographic materials, for example, as described in U.S. Pat. Nos. 3,152,904 and 3,487,075, or D. H. Klosterboer, “Dry Silver Photographic Materials” (Handbook of Imaging Materials, Marcel Dekker, Inc., page 48, 1991), known are silver salt photothermographic dry imaging materials (hereinafter, also referred to as photothermographic imaging materials or simply imaging materials) containing an organic silver salt, photosensitive silver halide and a reducing agent on a support. This silver salt photothermographic dry imaging material has an advantage capable of providing users with a system which is simpler and does not impair the environment because no solution type processing chemical is used at all.
Thus, the photothermographic imaging materials where image formation can be performed only by adding heat have come into practical use and rapidly prevailed in the above fields.
Typically in image diagnosis using imaging materials for the medical use, silver color tone formed by the development is an important factor which determines good or poor image quality. A silver ion reducing agent, a compound which forms a complex with silver ions, and a compound which bleaches fine silver nuclei which are sources of photographic fog produced on the surface of silver halide grains are contained in the silver salt photothermographic dry imaging material, and it is not easy to control developed silver shapes and maintain the image thereof after the thermal development. That is, not only the silver color tone immediately after thermally developing the imaging material must be controlled but also color tone changes must be reduced at a long term storage before the thermal development and at the storage of images after the thermal development. In earlier technology, these improvements have been attempted by controlling the developed silver shapes. For example, disclosed are the methods for reducing the changes of “color tone” under an atmosphere with high moisture by making particle sizes of the silver halide grains and fatty acid silver salt crystals small and controlling a “potency range” at the thermal development to the certain range (e.g., see Patent References 1 and 2).
Also, proposed are the improvement methods by activating photothermographic property by contrivance of fatty acid silver salt crystal structures (e.g., see Patent References 3 and 4), but it can not help being said that all methods are at insufficient levels in terms of realizing the stable silver color tone. Also disclosed is the method using leuco compounds which imagewisely produce yellow compounds by oxidation-reduction reaction at the thermal development, in combination with the certain silver ion reducing agent (e.g., see Patent Reference 5). However, the technology described in Patent Reference 5 is more excellent in improvement level of the color tone compared to the above technology which controls the developed silver shape, but has disadvantages that the photographic fog and deterioration of the color tone changes frequently occur at the long term storage and at the image storage probably because produced dyestuffs are unstable and further adversely affect the silver halide.
Also, in the light of effectively utilizing the silver which is a valuable resource, efforts to increase the maximum density on the imaging materials at an identical amount of the silver must be continued. A basic technical concept for this is to make individual developed silver small at the identical silver amount and make the particle sizes of photosensitive silver halide grains small. That is, the combination with so-called sensitization technology becomes essential. But when the individual developed silvers are made small, extents of optical scattering and absorption are changed and thus the silver color tone is changed. Thus, a new technology where the increase of maximum density, sensitization and color tone are compatible has been required.
These silver salt photothermographic dry imaging materials are characterized by making photosensitive silver halide grains provided in a photosensitive layer a photosensor, making an organic silver salt a source of silver ions, and in that images are formed by heat developing at 80 to 250° C. with a built-in reducing agent and no photographic fixing is carried out.
It is desirable to minimize an applied amount of silver which is a valuable resource in the dry imaging materials as well as in imaging materials in earlier technology. A basic technology includes making the photosensitive silver halide grains small. That is, individual developed silver produced after the heat developing becomes fine when a number of development initiation points is increased, and thus it is advantageous in terms of optical density because a ratio of a sectional area which the developed silver occupies per unit sectional area of the material is increased in the dry imaging material made of the same amount of silver. That is, it is possible to enhance a covering power value, increase the maximum density or accomplish saving silver. In technical examples included as the other technology for covering power enhancement, for example, disclosed is the technology to contain compounds which imagewasely produce chemical species capable of forming the development initiation points on and at vicinity of non-photosensitive aliphatic silver carboxylate and compounds similar thereto in the dry imaging material (e.g., see Patent References 6 and 7). In these technologies, improvement of covering power enhancement is observed, but they also have faults. Deterioration of image color tone is one example thereof. That is, as described in The Theory of the Photographic Process, fourth edition, pages 475 to 476 and Journal of Chemical Physics p-6755 to p-6759, 116 (2002), when sizes and shapes of the developed silver are changed, color tone of the developed silver is changed depending on absorption light and scattering light properties. When fine developed silver is produced, it mainly takes on a red tinge, and thus it often shifts from the color tone desired in the market. The reason why this color tone change is observed is thought to be that the number and ratio of fine silver clusters are increased, and in particular, it is noticeably observed at a high density area where the optical density is 2.0 or more. Thus, it has been difficult to simultaneously control the covering power enhancement and image color tone. Especially, in the silver salt photothermographic dry imaging material for the medical use, image quality improvement to enable more precise diagnosis is said to be one of extremely important properties, and as one example thereof, desired is the image color tone having the color tone where fatigue of the eyes is unlikely brought at observation.
At the same time, ideas have been made to improve the color tone. For example, a mix of photosensitive silver halide with different particle sizes and changes of halide compositions have been studied to control the shapes and sizes of the developed silver. Also, the ideas by the compounds referred to as color toning agents known to play a role as a silver carrier at the heat developing are disclosed (e.g., see Patent References 8 and 9), but it can not be said that significant improvement is obtained.
Also, the improvement of color materials has been attempted. For example, the improvement by leuco dyes is disclosed (e.g., see Patent References 10 and 11), but it can control only a part of hue, and further there is no description and suggestion for color tone improvement at the optical density of 2.0 or more. The improvement by coupler type coloring dyestuffs is disclosed (e.g., see Patent Reference 12), but the color tone control is difficult, slight deviance of the color tone occurs in every process, and reproducibility is poor.
Furthermore, when numerous fine silver clusters are present, so-called image storage stability is easily deteriorated such as the case where the imaging material after heating process is exposed under irradiated light. Specifically, many examples where silver image density and color tone are easily changed are observed. If residual sensitivity of the photosensitive silver halide after the heating process is low, this effect is reduced, but still it cannot be said that it is a sufficient level, and it is not preferable because the sensitivity at the regular exposure is also reduced. No reason other than the photosensitivity mentioned above is unclear, but for example, the finer the silver clusters are, the more the number increases, and this might easily become catalysis which reduces the silver of the residual silver salt. Or it might be because the fine developed silver per se is unstable for light and heat.
Therefore, strongly required is the technology where the color tone of images is improved with accomplishing the covering power enhancement and further the image storage stability after the heating process is improved.
Further, the photothermographic imaging materials (hereinafter, also referred only to as “photothermographic materials” or “imaging materials”) have already been suggested from the past. For example, they are described in U.S. Pat. Nos. 3,152,904 and 3,487,075, or D. H. Klosterboer, “Dry Silver Photographic Materials” (Handbook of Imaging Materials, Marcel Dekker, Inc., page 48, 1991), known are silver salt photothermographic dry imaging materials (hereinafter, also referred to as photothermographic imaging materials or simply imaging materials) containing an organic silver salt, photosensitive silver halide and a reducing agent on a support. This silver salt photothermographic dry imaging material has an advantage capable of providing users with a system which is simpler and does not impair the environment because no solution type processing chemical is used at all.
This photothermographic material is processed by a thermal development apparatus which adds stable heat to the photothermographic material to form the image, typically called a thermal developer. As mentioned above, in conjunction with the recent rapid prevalence, this thermal developer has been supplied in the market in large quantities. In the meanwhile, there has been problematic in that slipping property between the imaging material and a transport roller or processing members of the thermal developer changes, and transport failure and density unevenness occur. Also there has been problematic in that the density of the photothermographic imaging material varies with time. It has been found that these phenomena noticeably occur in the photothermographic imaging materials where image exposure is performed by laser light and subsequently the image is formed by thermal development. Also, recently, compaction of laser imager and acceleration of photographic processing have been required.
Therefore, property improvement of the photothermographic imaging materials becomes essential. For downsizing the thermal development processing apparatus, it is more advantageous to use a heat drum mode than to use a horizontal transport mode, but there has been problematic in that powder drop off, density unevenness and roller mark easily occur at the thermal development processing. Also, even when the rapid processing is carried out, to obtain sufficient density of the photothermographic imaging material, it is effective to use those with smaller mean particle size as silver halide to enhance covering power and use development accelerators such as hydrazine and vinyl compounds as shown in JP-A-11-295844 and JP-A11-352627. However, when these technologies were used, there was problematic in that density changes (printout property) with time after the thermal development processing became large and the silver color tone became extremely different compared to wet type X-ray films in earlier technology. Improvement technology of the printout property is disclosed in JP-A-2001-133925, regulation technology of the silver color tone is disclosed in JP-A-11-231460, JP-A-2002-169249, JP-A-2002-236334 and JP-A-2002-296729, and technology to inhibit the increase of photographic fog before and after the development is disclosed (see Patent References 13 to 15), but it could not be said that they were sufficient to solve all the above problems.